1. Field of the Invention
Embodiments of the present invention generally relate to a controlled gasification of agricultural waste products for utilization of the ash residue and gaseous exhaust.
2. Description of the Related Art
The disposal of waste or by-products from the processing of agricultural food crops often involves the burning of such by-products to create many problems for the food producing industry. By-products such as rice and peanut hulls, wood chips, cotton seed, etc. are tough, woody, and abrasive. Furthermore, such by-products are variable in density and have a high silica content. Incineration or combustion of such by-products is expensive, consumes large quantities of energy, and creates air pollution problems.
The controlled combustion or incineration of the foregoing type of waste or by-products or similar waste or by-products has heretofore been attempted with little success from either an economic standpoint or from an ecological standpoint. Because of feed density variation, overfiring or underfiring often occurs during combustion resulting in unstable heat generation and exhaust gas quality that is not satisfactory for heat recovery purposes. For example, the introduction of a feed with high silica content into the combustion chamber of a burner generates an exhaust stream with excessive fly ash, causing damage to and deterioration of boiler tubes because of silica related abrasiveness. Prior burners are also unable to control the degree of burn and therefore lack flexibility for control of the ash content of the combustion residue as a marketable product.
It is therefore an important object to provide an economical gasification system for a variety of feeds without requiring pretreatment or prior expensive processing and to accommodate a wide variation in feed bulk density.
Another object is to provide a gasification system for such waste products whereby the ash content of the combustion residue may be controlled and the fly ash content of its gaseous exhaust minimized.
A problem with typical combustion or gasification systems exists with respect to the accumulation of ash residue within the combustion or gasification system. The accumulation of the ash residue at non-discharging locations causes slagging, or hardening of the ash residue on the bed of the combustion or gasification system, due to overheating of the stagnant ash residue at the non-discharging locations. The gasification operation is often completely stopped (or at least slowed) by this build-up of ash residue at non-discharging locations. Thus, slagging of the ash residue is costly and time-consuming, as the operation of the gasification system must often be halted and personnel must be paid to remove the build-up of ash residue from the system. Furthermore, slagging of the ash residue is often costly because the parameters of the resulting ash residue product as well as the efficiency of the gasification system may be negatively impacted by the hardened build-up of the ash residue. The ash residue slagging problem is particularly acute when gasifying fuels having excessive amounts of potassium phosphates or low fusion temperatures, such as sewer sludge, distillery residues, sansa, straws, and other high alkaline fuels.
A further problem with currently utilized gasification systems involves the efficiency of the gasification process. The efficiency of the process is often compromised by the low temperature of the air within the gasification chamber. The efficiency and carbon conversion rate are often adversely affected by the air temperature and other variable operating parameters within the gasification system.
Yet a further challenge with the typical gasification system relates to the residence time of the fuel within the gasification chamber. The typical gasification system is only capable of gasifying a fixed quantity of fuel within the chamber at a time because of the fixed volume of the gasification chamber. Increasing the speed of the existing agitator arm to gasify more fuel per period of time causes the contents of the gasification chamber to become more volatile and as a result causes fuel or ash to entrain into the gas stream. Additionally, even when the gasification system is operated at the desired agitator arm speed, fuel or ash often entrains into the gas stream.
Also problematic with the current gasification systems is the bowing of the agitator arm. When the agitator arm within the gasification chamber is overheated, the agitator arm tends to bow upward and downward relative to the gasification chamber, compromising the efficiency and performance of the gasification system.
There is therefore a need for a gasification system where slagging of the ash residue at non-discharging locations within the gasification chamber is better controlled.
There is a further need for a gasification system with increased efficiency and greater carbon conversion as compared to currently utilized gasification or combustion systems.
Moreover, there is a need for a gasification system which is capable of maintaining the residence time of the fuel present in prior gasification systems even when slowing down the agitator arm to prevent entrainment of the fuel or ash into the gas stream. There is a further need for a gasification system which is capable of processing more fuel in a given time period without entrainment of the fuel or ash into the gas stream.
Finally, there is a need for a gasification system which reduces or eliminates possible damage to the agitator arm.